1. Field of the Invention
The present invention relates to a refrigerating machine oil, and a fluid composition for refrigerating machines and, more particularly, to a refrigerating machine oil useful for carbon dioxide refrigerant refrigerators, and a fluid composition for such refrigerators.
2. Related Background Art
The recent problem of ozone layer destruction imposed limitations on use of CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons), which have been used heretofore as refrigerants for refrigerator/air conditioner, and HFCs (hydrofluorocarbons) are being used as refrigerants instead of those restricted compounds. These HFC refrigerants, however, still involve problems of high global warming potential and others.
Thus use of natural refrigerants has been studied as substitute refrigerants for these fluorocarbon based refrigerants, and carbon dioxide (CO2) was suggested as one of such natural refrigerants. Carbon dioxide has the advantage in safety of being safe for the environment and, though it was out of the mainstream of refrigerants, it has been used heretofore as a refrigerant for refrigerators and others and thus research is recently going on to apply it as a refrigerant used in automotive air conditioners, room air conditioners, heat pumps for hot water supply, etc. incorporating open type refrigerant compressors or hermetic, electric refrigerant compressors.
Incidentally, refrigerating/air conditioning equipment is normally provided with a refrigerant recirculating system consisting of a refrigerant compressor, a condenser (gas cooler), an expander, an evaporator, and so on, and in this refrigerant recirculating system the refrigerating machine oil, which is lubricating oil for the refrigerant compressor, circulates together with the refrigerant in cycles. Accordingly, the refrigerating machine oil needs to satisfy various property requirements including miscibility with refrigerant, fluidity at low temperatures, stability, etc., in addition to lubricity. However, these properties of refrigerating machine oil are readily affected by types of refrigerants, and the properties were not satisfactory in lubricity and stability when the fluorocarbon based refrigerant refrigerators commonly used heretofore were operated together with the CO2 refrigerant.
Development has thus been made to develop novel refrigerating machine oils suitable for use along with the CO2 refrigerant, and refrigerating machine oils using a variety of base oils have been proposed. For example, Japanese Patent Kokai (Laid-Open) Publication No. H10-46169 discloses the refrigerating machine oil using an ether compound, such as polyalkylene glycol (PAG), as a base oil.